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45637-AC5
Fabrication of Nano/Microstructured Organic Films using Breath Figures
Breath figures form on cold solid or liquid substrates in contact with moist air. Breath figures that form on a solid surface demonstrate a range of self similar sizes, for after the initial growth regime, where droplet coverage is low and they grow independently of each other, beyond 40% coverage, coalescence sets in and growth occurs through coalescence dominated process. On the liquid substrates, at low droplet coverage, nearly monodisperse size drops are able to self organize as rafts and these rafts then assemble in patterns that are similar to two-dimensional crystals full of defects. Subsequently, coalescence sets in, where the pattern is rescaled, the growth rate is faster and the constant surface coverage of 55% is reached as in the case of solid substrates.
Breath figure like patterns appear on evaporating polymer solution, exposed to a blast of moist air. While on both solid and liquid substrates the latter stages in droplet growth are dominated by coalescence, in experiments done with polymer solutions, one is able to observe a highly ordered assembly of non-coalescing drops. These non-coalescing droplets eventually evaporate away leaving a film full of pores. The water droplets are usually monodisperse in size and can pack to the surface coverage approaching 0.90. In this study we elucidate the mechanism by which water drops nucleate, grow and assemble, the role played by solvent choice as well as humidity and speed of the blast of moist air and provide a theoretical framework which allows us to quantify the effect of these parameters on final assembly.
We model the evaporative mass loss problem from a flat film, by considering that the solvent diffuses through a polymer layer to arrive at the surface, where it is carried away by the flow of the blast of moist air. Using Fick's law with appropriate boundary conditions (mass transfer to air flow at top surface), we arrive at the rate of mass loss, and this compares quite well with the mass loss observed experimentally for the polystyrene dissolved in carbon disulfide.
The evaporation of solvent is accompanied by heat change associated with the latent heat of vaporization of the solvent and the heat flux at the surface associated with radiative, conductive and convective heat transfer. Next, by using heat diffusion equation in the flat polymer solution film, we numerically solve for the change in the temperature of the surface during evaporation, using the appropriate boundary conditions and solvent parameters. Our model that incorporates mass diffusivity, thermal diffusivity, heat of vaporization, specific heat as the solvent parameters is shown to capture the temperature change measured experimentally for polystyrene solution in carbon disulfide.
We note that the temperature first decreases below room temperature and as the evaporation of the solvent proceeds with decreasing rate, the temperature starts to rise back again. The drop in temperature and the width of the dip is controlled by the magnitude of thermal and mass diffusivity and by the losses due to airflow, related to flow velocity. We have calculated how the model profiles change for three different solvents, benzene, chloroform and carbon disulfide. By using the physical properties as input to the model of the solvent, we can find the temperature dip for any solvent of choice. This allows one to choose the solvent based on the observation of maximum evaporative cooling.
The kinetics of droplet nucleation, growth and assembly are determined by mass and heat fluxes of water vapor, and hence the variation of supersaturation (relative humidity>100%), which in turn are coupled to the corresponding fluxes for the evaporating solvent that we have already computed. In the present situation, the supersaturation is caused by the same phenomenon that causes mixing clouds that appear as fogging of breath in winter. The mixing of two sub-saturated parcels of air with different temperature and vapor pressure leads to a supersaturated mixture.
We have calculated how the supersaturation achieved depends on humidity of air flow as well as the impact of changing solvent from carbon disulfide to chloroform. We have derived the growth law for a single drop growing in this supersaturated mixture. In essence, we have derived the growth law for a single water droplet growing over an evaporating polymer solution, and illustrate that for this growth law, since smaller drops grow faster than large drops, the distribution grows to a monodisperse size. We have modeled how supersaturation depends on evaporation flux of solvent, it temperature and the flow velocity and humidity content of the blast of moist air. Thus for the first time, we are able to provide a quantitative model that accounts for influence of solvent parameters as well as flow parameters on the drop size and hence resulting pore size in polymer films.
